Aqueous polyurethane resin dispersions are being used as highly functional aqueous dispersions for paints, adhesives, textile processing and treating agents, paper treating agents, inks, and the like, because films obtained by drying the dispersions exhibit excellent performances in mechanical properties, durability, chemical resistance, abrasion resistance, and the like. Thus, in years to come, the polyurethane resin dispersions are expected to become increasingly more important from a viewpoint of environmental conservation, resource saving, safety, and the like. As a process for producing the aqueous polyurethane resin dispersion, there is known a process where a urethane prepolymer having terminal isocyanate groups is dispersed in water and chain extended with diamines and the like (a so-called prepolymer mixing process; refer to PTLs 1 to 3).
This is a process for obtaining a high molecular weight polyurethane resin by chain extending the urethane prepolymer with water and/or polyamine compounds in water, concurrently with dispersion or after dispersion. However, because this chain extension reaction is a heterogeneous reaction, the molecular weight distribution of the polyurethane resin becomes broad after the chain extension reaction and the film-forming property of the aqueous polyurethane resin dispersion obtained becomes worse. Thus, there has been a problem that performances (mechanical properties, durability, chemical resistance, abrasion resistance, and the like) of the resultant film deteriorate.
Furthermore, after the chain extension reaction, the terminals of the polyurethane resin become amino groups and, because of hydrophilicity thereof, there has been a problem that water resistance of the film obtained is lowered.